The recent portable electronic devices such as a foldable mobile phone and the like use a flex-rigid wiring board. Normally in such a wiring board, rigid or non-flexible portions and a flexible portion are joined to each other with a flexible substrate laid between them, and wiring pattern layers on the surfaces of the flexible and rigid substrates stacked together are electrically connected to each other via a plated through-hole conductive layer in the rigid portion as shown in FIG. 12 (as disclosed in the Japanese Patent Application Laid Open No. 90756 of 1993, for example).
Also, there has been proposed a flex-rigid circuit board in which a multilayer rigid substrate has formed at either end thereof a cut in which an end electrode of a flexible substrate is fitted, the end portion of the flexible substrate is laid between the outermost sides of the rigid substrate and the electrode of the rigid substrate and that of the flexible substrate are electrically connected to each other (as disclosed in the Japanese Patent Application Laid Open No. 170076 of 1995, for example).
However, the wiring board in which a flexible substrate and rigid substrate are electrically connected to each other through a plated through-hole with the above-mentioned conventional wiring-board technology is not advantageous in that when it is used with a frequency of higher than 1 GHz, signal propagation is delayed and high-speed signal transmission is not stable. Especially when the wiring board is used with a frequency of higher than 5 GHz, the signal propagation is more seriously delayed and instability of the high-speed signal transmission is greater.
Also, the wiring board produced with the conventional wiring-board technology was found, as a result of the reliability test made in the cooling/heating cycle test, to have the electrical connection thereof deteriorated. That is, for the electrical connection through the plated through-hole in the flexible and rigid substrates, the conductive layer in the through-hole is formed by plating. Therefore, the conductive layer varies in thickness so that some of connecting terminals are not connected, namely, so-called “open” terminals are resulted.
Further, since the conventional flex-rigid wiring board is formed either by fixing a flexible substrate to an outermost rigid substrate with an anisotropic adhesive or by laying a flexible substrate between both outermost rigid substrates, so it cannot be designed thinner.